Currently, the tubes are constituted by an inner layer which defines the passage conduit for the fluid, and by one or more outer layers superposed to the inner layer and made integral therewith.
Typically, the outer layers can be constituted by fabric, rubber, metallic spirals for reinforcement and/or protection, various filling materials, etc. . . .
Depending on the applications, one or more layers constituting the tube may be made of a conductive material, to discharge any electrostatic charges generated with the passage of the fluid. This solution is necessary when the tubes are destined to be used in environments in which flammable and/or explosive materials are present, or when the fluid to be conveyed is flammable and/or explosive.
In regard to the inner layer, it is currently made of silicone, or in fluoridated polymers such as FEP and PTFE, and its thickness may vary between 1 and 2 mm.
In many applications (in particular in the industries mentioned above), the inner conduit for the passage of the fluid must be white. This requirement is well met by conduits with thickness exceeding 1 mm made of materials such as PTFE which by nature are translucent. Given their high thickness, the colour of the conduit appears to be white.
The choice of thickness of 1 or 2 mm is in any case dictated by reasons of a mechanical nature, given that, at least to date, it has not been possible to produce conduits of smaller thickness having sufficient mechanical resistance.
The production of tubes of the type described above starts from the inner conduit which is first of all externally subjected to a process called cementing whose purpose is to increase its “roughness” (in this case meant more at the molecular than macroscopic level), then treated with a product that acts as an adhesive and binds to the outer, “rough” surface of the inner layer itself.
Subsequently, the outer layers are applied in succession according to the needs dictated by the intended use of the tubes.
Said known type of tubes, however, has a high mechanical rigidity, due mainly to the high thickness of the inner conduit, which substantially prevents bending the tube.
This prevents use of such tubes in combination with movable devices, which in some cases can be the filler heads of filling machines.
Another known type of tubes is represented by tubes lacking outer coating, thus constituted by the inner conduit only.
Although said tubes too are made of the same materials indicated for the inner conduit of the multilayer tubes, recently others have been developed, made of modified fluoridated polymeric materials, known as MFA (polytetrafluoroethylene-perfluoromethylvinylether) and PFA (polytetrafluoro-ethylene-co-perfluorine(alkylvinylether)).
These two polymeric materials have such characteristics as to allow making tubes with a thickness of even 0.2 mm, which nonetheless assure excellent mechanical strength, excellent impermeability, and can convey even highly aggressive fluids, without releasing or absorbing any element.
Such materials are transparent and with the cementing process they take on a dark colour; thus, the pigmentation of the tubes during their production is always necessary to make them white.
While such materials do have advantageous characteristics, heretofore they have not been considered suitable to obtain multilayer composite tubes, since at present no effective cementing operation can be performed on pigmented elements made of MFA or PFA. A cementing operation is indispensable to allow the adhesion of the outer layers to the inner conduit.